For example, in the viscous coupling, a plurality of inner metal discs movably disposed on a driving shaft and a plurality of outer metal discs fixed on a driven shaft with predetermined spacings are combined together alternately and are accommodated in a housing, which fills a viscous operating oil for torque transmission. Under such an arrangement, a shearing force, i.e. a shear torque, is generated in the discs due to the difference in the rotation speed between the driving shaft and the driven shaft, whereby the torque is transmitted to the driven shaft.
As the operating oil for viscous coupling, organopolysiloxane such as dimethylpolysiloxane and methylphenylpolysiloxane, generally having high viscosity index, is used. The organopolysiloxane is easily subjected to thermal degradation or gelation when used at high temperature because of its poor thermal stability, whereby its viscosity increases and its torque transmission ability cannot be maintained stably.
For improving the thermal stability of organopolysiloxane, it has been proposed to add an antioxidant such as iron octoate, aromatic amine derivatives and ferrocene derivatives in organopolysiloxane. When the composition comprising organopolysiloxane and the antioxidant was practically used in a viscous coupling, however, the increase in viscosity was observed.